2. The conventional production of hydrogen peroxide product involves a two-step process in which a hydrogen donor solvent, ethyl anthraquinone (EAQ), is first hydrogenated and then oxidized with oxygen to form the hydrogen peroxide product. In some hydrogen peroxide manufacturing facilities, the hydrogenation step is carried out in a fixed bed reactor utilizing a palladium-on-alumina or similar catalyst. A typical catalyst may contain 0.28 to 0.33 wt % palladium on a large pore alumina support. The useful life expectancy of the catalyst is about two years, after which its activity drops to about 30% of its original (fresh catalyst activity) condition or level. It is believed that this catalyst deactivation is caused by deposition of high molecular weight organic materials formed from the polymerization of EAQ on the active sites of the catalyst, and/or by gradual agglomeration of the palladium to larger particles or clusters on the catalyst. Spent palladium/alumina catalysts are presently being regenerated using a simple “wash-burn” procedure, in which the catalyst is first extracted with an organic solvent to remove any soluble material deposits; then, the spent catalyst is subjected to a controlled carbon burn-out step at about 850° F. temperature in air. The high temperature regeneration treatment may also promote undesirable agglomeration of the palladium to larger particles on the catalyst support. Thus, it is difficult to successfully regenerate the used catalyst back to near 100% of its original activity. In fact, the simple wash-burn procedure can usually restore the used catalyst to only about 70% of its original or fresh activity level. Such “wash-burn” catalyst regeneration procedures have been disclosed by various U.S. and foreign patents. For example, U.S. Pat. No. 4,148,750 to Pine discloses a process for redispersal of noble metals on used supported zeolite-containing catalysts. U.S. Pat. Nos. 4,454,240 and 4,595,666 to Ganguli disclose method steps for regenerating used catalysts by dilute acid treatment to remove undesired metal deposits followed by carbon burn-off at increased temperature levels. Also, U.S. Pat. No. 5,188,996 to Huang et al discloses redispersion of noble metal such as platinum on low acidity support such as silica by contacting with chlorine and oxygen at low pressures.
An overall objective of the present invention is to overcome the limitations inherent in prior art spent catalyst regeneration processes and provide a process that will restore spent catalyst activity to a level at or near that of fresh metal-on-solid support catalyst. A further objective of the invention is to provide a regeneration process with the foregoing capabilities that will be applicable generically to spent solid supported metal catalysts, regardless of composition. A particular objective is to improve the economics of the hydrogen peroxide production process by increasing the activity and service life of the palladium/alumina catalyst, as well as that of other similar supported noble metal catalysts.
Based on an understanding at the molecular level of the apparent catalyst reaction and deactivation mechanism, the surface structure of the catalyst support material, and the exposition of palladium crystal clusters thereon, an effective procedure has been developed for regenerating and enhancing used palladium/alumina catalyst to an activity level significantly higher (90% or more) than that achieved by the current “wash-burn” procedure. The procedure is applicable to catalysts comprising both noble and non-noble metal-on solid support wherein the solid support comprises any solid material useful as a support for solid metal catalysts.